To appreciate the significance of the subject invention, it is necessary that certain terms used herein be defined. In this regard, the terms "ultrafiltration" and "ultrafilter" are commonly used, and will be used herein, to describe a filtration process and a filter respectively having the ability to remove particles as fine as about 0.001 micrometers up to about 10 micrometers (microns), a range of particle sizes commonly referred to as "ultrafine". Ultrafiltration media with very fine pore sizes are recognized as useful for filtering ultrafine particles from various liquid media. Unfortunately, ultrafilters in general have efficiencies below 100 percent in the range of below 0.1 micrometers.
The term "efficiency", as used herein, means the the ability of a filter medium to remove particulate contaminant of a given type, that is, it is the percent of that particular type particulate contaminant which is unable to pass through the filter. For example, to refer to a filter medium with an efficiency of 100 percent for a given type particle means that the effluent from that filter contains none of that particular species of particulate contaminant, whether that species is characterized by size alone, the electrocharge on the particle, another property of the particle, or a combination of such characteristics. As used herein the term "essentially absolute efficiency" means the ability to remove a particular particulate contaminant at the 99.99 percent level or better. Correspondingly, the term "substantially free" of a particular contaminant means that the level of the particular contaminant in the effluent from the filter system has been reduced by 99.99 percent of its influent concentration and, in some cases, to substantially lower levels.
The function of a filter is the removal of suspended particulate material and the passage of the clarified fluid medium (filtrate or effluent). A filter can achieve fluid clarification by different mechanisms. Particulate material can be removed through mechanical sieving, wherein all particles larger than the pores of the filter are removed from the fluid. With this mechanism, filtration efficiency is controlled by the relative size of the particulate contaminant and the physical pore size of the filter. The efficient removal of very small particles, e.g., less than 0.1 micrometer in diameter, requires ultrafilters with very small pore sizes. Such fine pore filters tend to have the undesirable characteristics of high pressure drop across the filter, reduced dirt capacity and shortened filter life, resulting in an inefficient and uneconomical means for providing high level purification of contaminated fluids. These problems are exacerbated by the almost invariable tendency for the skinned membranes, typically used as ultrafilters, to have pin holes providing the highly undersirable result (for some applications) of allowing larger size particles to penetrate the filter, thereby contaminating the downstream filtrate with, e.g., bacteria, rendering such ultrafilters incapable of being used for providing a sterile filtrate. Such membranes are poorly suited, then, wherever essentially absolute efficiency for the removal of ultrafine particulate material, i.e., wherever complete removal of incident bacteria is required.
A filter may also remove suspended particulate material by adsorption onto the filter surfaces, that is, the surfaces of the pores in the filter. Removal of particulate material by this mechanism is controlled by the surface characteristics of (1) the suspended particulate matter, and (2) the filter. Most suspended solids which are commonly subjected to removal by filtration are negatively charged in aqueous systems. This feature has long been recognized in water treatment processes where cationic flocculating agents, oppositely charged to the suspended matter, are employed to improve settling efficiencies during water clarification.
Colloid stability theory can be used to predict the interactions of electrostatically charged particles and filter surfaces. If the charges of the suspended particles and the filter surfaces are of like sign and with zeta potentials of greater than about 20 mV, mutual repulsive forces will be sufficiently strong to prevent capture by adsorption. If the zeta potentials of the suspended particles and the filter surfaces are small or, more desirably, of opposite sign, the particles will tend to adhere to the filter surfaces with high capture efficiencies. Most particles in the suspensions encountered in industrial practice have a negative zeta potential. Thus, microporous filters characterized by positive zeta potentials are capable, in a large number of industrial applications, of removing particles much smaller than the pore diameters of the filter through the mechanism of electrostatic capture. As a result, the high pressure drops, reduced dirt capacity and shortened filter life encountered with a filter operating strictly as a mechanical sieve can, to a large extent, be avoided.
The drawback of a filter operating as an adsorption filter by virtue of interaction between the particulates being filtered and the surfaces of the filter medium is that such a filter with a given zeta potential (positive or negative) will not capture similarly charged particulates smaller than the physical pore size of the filter, due to the mutually repulsive forces of the particles and the filter surfaces. Thus, for example, fine asbestos particles which carry a positive charge will not be removed from fluid media passed through a filter medium having a positive zeta potential except by a sieving mechanism as discussed above.
Similarly, with particulates of little or no charge, e.g., some bacteria and endotoxins as well as some other particulates, the only assured way of removing these materials by a filtration process is by a sieve mechanism. The conundrum has been that a filter with physical pore sizes fine enough to capture very fine particulates by a sieve mechanism quickly develops a high pressure drop as particulate matter clogs these fine pores, the limited capacity of the filter is quickly reached and the filter life is shortened to the point that economic ultilization of such filters is restricted.
The present invention is directed to novel filter systems capable of greatly enhanced filtration efficiency over a broad pH range and with a wide variety of particulate contaminants, including ultrafine particulates, particularly very fine negatively charged particles, very fine positively charged particles, and substantially neutrally or uncharged particles. The filter systems of this invention have extended lives relative to conventional fine pored ultrafilters, such as skinned membranes, because of the unique combination of filter media which serves to protect the very fine pored, downstream, last chance or final filter.
Filter systems of the present invention are also capable of delivering high purity effluent water rapidly after the onset of filtration, the purity level being such that the resistivity of the effluent water rapidly reaches the theoretical resistivity of water, i.e., greater than 14 megaohms/cm. This ability makes filter systems of this invention particularly desirable for the filtration of aqueous fluids employed in microelectronics manufacture where ever increasing packing densities in microcircuits are forcing the manufacturers to seek filtration systems with the ability to remove very fine contaminants from their processing liquids. Further, filter systems of this invention have the capability of removing very fine contaminants from process liquids, such as the water used by electronics manufacturers to make microcircuits, without the large capital investments presently typical for installation of conventional ultrafiltration systems.